Induction motors are widely used in industrial applications owing to their low cost and high efficiency and reliability, among other benefits. A typical induction motor includes a stationary member, or stator, having a plurality of windings thereon and a rotating member, or rotor, rotatably disposed within the stator. Applying a sinusoidal or alternating voltage to the stator windings induces a rotating magnetic field that causes the rotor to rotate. Induction motors typically operate on single-phase and three-phase voltages, although two-phase induction motors are also available.
Various techniques exist for starting an induction motor. One such technique is direct on-line (DOL) starting in which the terminals of the motor are directly connected to a power supply, which is typically connected to the AC main. With DOL starting, a three-phase induction motor may draw between five to eight times its rated full load current immediately after startup, known as “locked rotor” current. As the motor accelerates, the load current drops slightly until the motor reaches about 70% to 80% of its full speed, after which the load current drops more rapidly toward the normal running current of the motor. The time it takes the motor to reach full speed is called the “startup” time and may last several seconds, depending on the size of the motor and the inertia of the driven system.
Most motor overload protection devices that protect induction motors from current overload take the above startup time into account before disconnecting power to the motors. These devices, which may include circuit breakers, overload relays, and other types of circuit interruption devices, typically delay tripping for some number of seconds based on the “trip class” of the device to accommodate the startup time. A higher trip class may be used for motors that have higher inertia loads, and so on, with some overload protection devices allowing up to 30 seconds for the motor to reach full speed before tripping, known as a “long start.”
Sometimes a motor is stalled or otherwise unable properly to start due to excessive load, motor damage, or other reasons. When this happens, the motor stills draws locked rotor current, but does not rotate or start because the motor has stalled or jammed. Stalled starts, however, are not typically differentiated from long starts in existing overload protection devices. These overload protection devices typically allow a stalled motor to continue drawing locked rotor current until the designated startup period expires before tripping. Such an extended interval of locked rotor current in the motor windings may damage the motor and power supply circuits, generate excessive heat, especially if the motor cooling system is not working, which is often the case when the motor is stalled, and require longer cool down periods before subsequent restarts are allowed.
To address the above problem, certain high end overload protection devices use analysis of phase shifts that may exist between supply voltage and motor current to detect a stalled start. Other overload protection devices use a tachometer to directly measure motor speed and thereby detect a stalled start. Motor speed may also be estimated, for example, using rotor bar harmonics and the like. However, in addition to increasing complexity and cost, some of these techniques, such as the use of rotor bar harmonics to estimate motor speed and the like, do not function during transient conditions like during a motor start.
Thus, a need exists for an improved way to detect and protect induction motors from stalled start conditions.